数据库教程:mysql慢查询优化之从理论和实践说明limit的优点分享

很多时候， 我们预期查询的结果最多是1条记录数据， 那么这个时候， 最好用上limit 1,  当查到这条数据后， mysql会立即终止继续查询， 不进行更多的无用查询， 从而提升了效率。

我们来实际测试一下， 在一个拥有10万的mysql表中， 查找lily的分数（假设系统中只有1个lily, 而我们预期也只需要这条数据）。为了显示出时间的差别， 我并不对表的name字段建索引。

先看看表结构：

mysql> show create table tb_province; +-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | tb_province | CREATE TABLE `tb_province` ( `id` bigint(10) unsigned NOT NULL AUTO_INCREMENT, `name` varchar(32) NOT NULL, `score` int(10) unsigned DEFAULT '0', `x` int(10) unsigned DEFAULT '0', `x1` int(10) unsigned DEFAULT '0', `x2` int(10) unsigned DEFAULT '0', `x3` int(10) unsigned DEFAULT '0', `x4` int(10) unsigned DEFAULT '0', `x5` int(10) unsigned DEFAULT '0', `x6` int(10) unsigned DEFAULT '0', `x7` int(10) unsigned DEFAULT '0', `x8` int(10) unsigned DEFAULT '0', `x9` int(10) unsigned DEFAULT '0', `x10` int(10) unsigned DEFAULT '0', PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=124178 DEFAULT CHARSET=latin1 | +-------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)

我们打开set profiling=1;的开关，执行mysql语句来对比：

mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.03 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.03 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.04 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.02 sec) mysql> select score from tb_province where name='lily'; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.03 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.01 sec) mysql> select score from tb_province where name='lily' limit 1; +-------+ | score | +-------+ | 100 | +-------+ 1 row in set (0.00 sec)

可见，我们针对是否采用limit 1进行了5次对比测试， 来看看结果吧：

mysql> show profiles; +----------+------------+---------------------------------------------------------+ | Query_ID | Duration | Query | +----------+------------+---------------------------------------------------------+ | 5 | 0.02686000 | select score from tb_province where name='lily' | | 6 | 0.02649050 | select score from tb_province where name='lily' | | 7 | 0.03413500 | select score from tb_province where name='lily' | | 8 | 0.02601350 | select score from tb_province where name='lily' | | 9 | 0.02785775 | select score from tb_province where name='lily' | | 10 | 0.00042300 | select score from tb_province where name='lily' limit 1 | | 11 | 0.00043250 | select score from tb_province where name='lily' limit 1 | | 12 | 0.00044350 | select score from tb_province where name='lily' limit 1 | | 13 | 0.00053200 | select score from tb_province where name='lily' limit 1 | | 14 | 0.00043250 | select score from tb_province where name='lily' limit 1 | +----------+------------+---------------------------------------------------------+ 14 rows in set, 1 warning (0.00 sec)

可见，采用limit 1后， mysql语句的效率确实提升很多。 当表更大时， 效率提升会更加明显。 

我们已经从理论和实践的脚本都说明了limit的优点， 所以， 建议是：在可用limit的时候要用limit （当然， 如果结果是多个，肯定不能limit 1啊）

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